Leaching Release of Heavy Metals Cadmium, Cobalt, Chromium, Copper, Nickel, Lead, and Zinc From Slag-Amended Sandy Soils.

Zinc smelting activities in Guizhou, China produced a huge amount of slags piling up on roadsides or hill slopes. Under surface condition (such as weathering or rain water washing), slag grains are frequently scattered into soils, resulting in accumulation of heavy metals in soils. When subjected to acid deposition, these slag contaminated soils could lead to the release of heavy metals into surface water bodies. A column leaching study was designed to test the potential release of the heavy metals under simulated acid rains. Two slags with different weathering age were amended in a sandy soil at levels of 0, 0.5%, 1.0%, 2.5%, and 5.0%, respectively. After thoroughly mixed and soil moisture adjusted to 70% field water holding capacity, the mixtures were incubated for 120 days at room temperature. The incubated soils were then packed into a column and subjected to acid leaching (pH 4.5) at intervals of 1, 3, 7, 14, 28d, respectively. After passed through a 0.45 μm membrane, collected leachate was analyzed for heavy metals cadmium, cobalt, chromium, copper, nickel, lead, and zinc, and cations using ICP-AES and anions using IC. It was observed that heavy metal concentrations in the leachate had significantly negative correlations with leachate pH, and were positively related to slag amendment rate. Soils amended with the younger age slag resulted in more heavy metals in the leachate due to their higher contents in the younger age slag. After 5 leaching events, leaching percentages (cumulative amount in leachate divided by Mehlich 3 extractable quantities in amended soils) of Cd, Co, Cr, Cu, Ni, Pb, and Zn from soils amended with older age slags were up to 5%, 14%, 10%, 2%, 8%, 2%, and 10%, whereas, those from younger age slag amended soils accounted for 1.3%, 6.5%, 5%, 1%, 4%, 0.1%, and 1.3 %, respectively. Release of heavy metals from slag amended soils under acidic condition was jointly affected by age of slags, mineral composition of slags, and slag amendment rate in soils. 

Key words: Column leaching; Soil amendment; Mineral composition; Slag age